The present invention relates to a front end module mainly including an antenna switch available to mobile communications apparatus such as a cell phone and an automobile phone.
A high frequency circuit of related art mobile communications apparatus includes an antenna switch or selectively connecting an antenna to a transmitter circuit or a receiver circuit, a surface acoustic wave filter (hereinafter referred to as a SAW filter) as a band-pass filter for filtering a signal received via the antenna switch and an amplifier circuit for amplifying a received signal that has passed the filter.
The internal circuit of an analog IC used in the amplifier circuit of a receiver circuit generally includes a differential amplifier circuit. The input terminal of an analog IC is usually of a balanced system for input a received signal as a potential difference between two terminals. Thus, it is desirable to use a balanced SAW filter as the SAW filter.
However, it is difficult to provide a good balance to obtain sufficient characteristics in a balanced SAW filter. This is because a more minute pattern design is required as the frequency band used gets higher and sensitivity to a line length becomes higher as the frequency elevates, that is, it is difficult to provide good balance characteristics in a SAW filter used in a higher frequency. Therefore, a common practice is to use an unbalanced-output SAW filter and a balanced-to-unbalanced line transformer (called a balun) is provided between the SAW filter and an amplifier circuit in order to convert an unbalanced signal to a balanced signal. This is because a balun does not require a very high element sensitivity and it is relatively easy to provide a good balance. In this case, an impedance across balanced input terminals of the analog IC in the receiver circuit is determined per IC so that the balun used conforms to the particular impedance. The Japanese Patent Publication No. Hei. 10-32521 discloses a front end module including a multilayer substrate and a surface-mounted components, the front end module incorporating a SAW filter.
As described in the Japanese Patent Publication No. Hei. 10-32521, in a front end module that incorporates a SAW filter on a multilayer substrate, the output impedance is determined by the characteristics of the incorporated SAW filter. Thus, a SAW filter of a desired impedance is essential to the front end module.
In case an unbalanced SAW filter is used, the user must provide a balun between the receiving IC and a front end module.
Using a different SAW filter each time the impedance is changed requires a matching design of the entire front end module. This needs the corresponding time.